This invention relates to a method and apparatus for laser ignition.
Laser light has been used to initiate the ignition of fuel/oxidizer mixtures by use of laser-spark, air-breakdown ignition methods in which a single, high peak power laser light pulse from a Q-switched laser is used to initiate fuel ignition by generating high temperatures and an ionization plasma. These laser ignition methods and apparatuses are generally unreliable except within narrow ranges of fuel parameters such as fuel/oxidizer ratios, fuel droplet size, number density and velocity within a fuel aerosol, and initial fuel and air temperatures.
After initial ignition, sustaining ignition of fuel/oxidizer mixtures is typically accomplished by use of a laser light pulse from a Q-switched laser with a pulse width and pulse energy which will provide the peak power density required to initiate plasma formation and to satisfy concurrently the need for time-averaged power for sustaining the ignition. This requires fragile, bulky laser excitation sources such as flashlamps or laser diodes which are often difficult to fit proximate to fuel combustion zones, particularly in fuel combustion zones in places such as aircraft engines. Fuel combustion zones are usually harsh, mechanically adverse environments that necessitate sturdy design or frequent replacement of ignition devices subjected to those environments.
Thus there is still a need for a laser ignition process which can reliably ignite gaseous or aerosol fuel mixtures within a broad range of parameters such as fuel/oxidizer ratios, fuel droplet size, number density and velocity within a fuel aerosol, and initial fuel temperatures as well as a need for means for ignition within small spaces under mechanically adverse conditions.
Therefore, it is an object of this invention to provide a reliable ignition method and apparatus.
It is another object of this invention to provide a method and apparatus for laser ignition of gaseous or aerosol fuel mixtures within a broad range of parameters such as fuel/oxidizer ratios, fuel droplet size, number density and velocity within a fuel aerosol, and initial fuel temperatures.
It is yet another object of this invention to provide an economical method and apparatus for laser ignition of gaseous or aerosol fuel mixtures.
It is a further object of this invention to provide a method and apparatus for laser ignition of gaseous or aerosol fuel mixtures within small spaces under mechanically adverse conditions.
It is yet a further object of this invention to provide a method and apparatus for elimination of fragile and bulky laser excitation sources such as flashlamps or laser diodes from fuel igniting lasers located proximately to fuel combustion zones.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims which are intended to cover all changes and modifications within the spirit and scope thereof.
To achieve the foregoing and other objects, and in accordance with the purposes of the present invention, as embodied and broadly described herein, there has been invented a fuel ignition apparatus and method in which an excitation light source is used to activate one or more small ignitor lasers located remotely from the excitation light source and more proximately to one or more fuel combustion zones. Using a separate excitation light source to pump the lasers which ignite the fuel eliminates the need for large, heavy, fragile or complex excitation sources in the harsh operating environments of fuel combustion chambers. Durable, reliable, economical ignition of gaseous or aerosol fuel mixtures can be accomplished with the invention apparatus and method. In a third embodiment of the invention, light beams from a remote excitation light source are transported by an optical fiber to at least one ignitor laser which is positioned to direct a beam into a focal point in an aerosol spray or cloud of a combustible fuel. The excitation light source is used to provide a sequence of two low peak power pulses separated by a short time interval. After the ignitor laser is pumped by the first of the pulses from the excitation light source, the ignitor laser outputs a short duration high peak power pulse which breaks down and ignites the fuel. Before the ignitor laser Q-switch is reset, the second of the long duration low peak power pulses from the excitation light source re-energizes the ignitor laser to produce a long duration low peak power pulse in the output of the ignitor laser. The long duration low peak power pulse is also focused into the focal point in the aerosol spray or cloud of the combustible fuel to sustain the plasma formed by contact of the short duration high peak power pulse with the fuel.
Any of the invention apparatuses can have an optical switching feature positioned to receive output of a single excitation light source and to direct beams in an ordered sequence or in a random sequence through beam splitters or multiplexing devices into a plurality of optical fibers, each of which is associated with an ignitor laser. When it is desired not to use a fiber optic delay line or a second optical fiber direct to the fuel combustion zone, as is the case of the third embodiments, the multiplexing feature is positioned to receive output of a single excitation light source and to direct beams in an ordered or random sequence into single optical fibers which are each connected to an ignitor laser.